In an existing Multiprotocol Label Switching (MPLS) domain, the Resource Reservation Protocol-Traffic Engineering (RSVP-TE) signaling protocol is generally used to establish an LSP.
In an MPLS network, each node is configured with the RSVP-TE protocol and is configured with an interface with a need to run the RSVP-TE protocol to run the RSVP-TE protocol. Between two adjacent nodes in the MPLS network, the RSVP-TE protocol maintains a peer relationship between the nodes by using a Hello packet. When an LSP is established, the RSVP-TE protocol sends, from an ingress, a path message downstream and hop by hop and then sends, from an egress, a reservation (Resv) message to the ingress. When receiving the Resv message, each hop on a link reserves a bandwidth resource for the LSP, allocates a label, and sends the label to an upstream node by using the Resv message. After the Resv message arrives at the ingress, the RSVP-TE protocol regards that the entire LSP is successfully established.
By default, the RSVP-TE protocol sends a Hello message periodically every 3 s, sends the Path message to the downstream node periodically every 3 s, and sends the Resv message to the upstream node. It is evident that, in order to maintain link and LSP status information, the RSVP-TE protocol needs to send lots of soft state packets such as Hello, Path, and Resv, which causes heavy load to a node and reduces the number of RSVP-TE LSPs that the node is capable of supporting.